Ambient backscatter is a radio communication principle that allows communication devices to communicate respectively to transfer data or pieces of information by backscattering ambient radiofrequency (RF) electromagnetic radiation which has been generated by means of a radio radiation source external to the communication devices.
When transferring information by means of ambient backscatter communication a time varying backscattering of electromagnetic radiation is achieved by changing the impedance of an antenna arrangement under the influence of electromagnetic radiation which is present “within the air”. This means that at least a part of the electromagnetic radiation “within the air” impinges onto a transmit communication device which is supposed to transmit the pieces of information to a receive communication device.
The physical principle of ambient backscatter communication can be understood when recognizing that generally when a wave encounters a boundary between two media that have different impedances/densities, the wave is reflected back at least partially. The amount of reflection is typically determined by the difference in the impedance/density values. In case of acoustic waves the acoustic impedance is the relevant property of the two media having the boundary in between. In case of electromagnetic waves the electric impedance is the relevant property of the media defining the boundary. By modulating the electrical impedance (at the port) of the antenna arrangement of the transmit communication device in accordance with pieces of information to be transmitted one automatically modulates the amount of RF energy that is scattered towards the receive communication device. Be demodulating the received scattered electromagnetic radiation one can retrieve the pieces of information.
Another approach for understanding the physical principle of ambient backscatter communication is by considering that the electromagnetic radiation being present “within the air” is attenuated with a time varying pattern by means of the transmit communication device. Thereby, the time varying pattern is associated with the pieces of information which are supposed to be transferred. The receive communication device recognizes the corresponding time varying attenuation of the “within the air” electromagnetic radiation and retrieves, based on the recognized time pattern, the pieces of information.
By contrast to known transponder technologies, i.e. Radio Frequency Identification (RFID) technology, ambient backscatter communication does not require a transmit system, which emits electromagnetic waves. By contrast thereto, ambient backscatter communication is based on a modulated attenuation of an electromagnetic wave being within space. This has the effect that with ambient backscatter communication both the transmit communication device and the receive communication device do not require any power for generating electromagnetic waves. Only a minimum of power is needed for generating (by the transmit communication device) and for receiving (by the receive communication device) the time modulated attenuation.
WO 2015/123341 A1 and US 2015/0311944 A1 disclose devices among which pieces of information can be transmitted by means of an ambient backscatter communication.
One “apparatus problem” with ambient backscatter communication is that at the transmit side there must be a strong electromagnetic coupling between (a) the (“driving”) electromagnetic radiation being present in space respectively “in the air” and (b) an antenna arrangement of the receive communication device for providing an efficient modulation of attenuation. On the receive side at the receive communication device electronic circuits, which have to perform in particular a demodulation procedure, are subjected to the electromagnetic radiation being present in space. This typically results in an unwanted electromagnetic interference (EMI) which may negatively impact the operation of the electronic circuit, which in turn diminishes the quality of the received signals representing the transferred pieces of information.
Further, in particular with small communication devices (the electronic components of) the electronic circuits provide for an unwanted shielding of the antenna arrangement at least with respect to certain directions. On the transmit side this shielding of the antenna arrangement may affect the “driving” electromagnetic radiation being present in space and the backscattered (time modulated) radiation. On the receive side a shielding of the antenna arrangement yields a (time independent) attenuation of the ambient backscatter radiation which can be picked up. These shielding impacts increase when the electronic circuits are located close to the antenna structures. On the other hand, if the electronic circuits are located “far away” from the antenna arrangement, due to a correspondingly large length of electric wiring between the respective electronic circuit and the antenna structure, the SNR decreases. As a consequence, the quality of ambient backscatter communication is reduced.